The present invention relates to a process for the preparation of a dilithiated initiator usable in anionic polymerization, a process for the preparation by anionic polymerization of a living diene elastomer which is provided with a carbon-lithium group at each of its chain ends, and a process for the preparation of a copolymer having three blocks starting from said living diene elastomer.
The preparation of dilithiated polymerization initiators has been amply described in the literature. The main difficulty which is encountered in this preparation is to obtain initiators which are effectively bifunctional, that is to say are such that they permit equi-reactivity of the carbon-lithium sites during anionic polymerization reactions using these initiators.
The first methods of preparing such dilithiated initiators involved the coupling of radical anions.
Mention may be made, for example, of the preparation of 1,4-dilithiotetraphenyl butane, from a precursor consisting of 1,1-diphenylethylene (L. J. Fetters and M. Morton, Macromolecules 2, 453, 1969).
The main disadvantage of this first method of preparation lies in the necessity of using metallic lithium, which represents a difficulty when extrapolating to an industrial scale. Furthermore, the precursor used is very expensive and addition of monolithiated compounds to a di-unsaturated compound.
Examples of such di-unsaturated compounds include compounds bearing two diphenylethylene units, such as 1,3-bis(1-phenylethenyl) benzene (L. H. Tung and G. Y. S. Lo, Macromolecules 27, 1680, 1994).
To this end, international patent specification WO-A-89/04843 discloses a process for the preparation of a dilithiated initiator consisting of reacting, in cyclohexane, secondary butyllithium with 1,3-bis(1-phenylethenyl)benzene in stoichiometric quantities. In order to make the initiator thus prepared have a bifunctional nature during polymerization, it should be noted that a polyamine must later be introduced into the polymerization medium.
One major disadvantage of this second method of preparation lies in the lack of availability of the precursors for use on an industrial scale.
The use of aromatic di-unsaturated type precursors (such as divinylbenzene, or alternatively 1,3-diisopropenylbenzene) makes it possible to overcome this disadvantage.
Thus, the reaction between the 1,3-diisopropenylbenzene and the secondary butyllithium has been widely studied. Two examples of implementation of this reaction may be mentioned. In the first example, R. P. FOSS et al. (R. P. Foss et al., Macromolecules 10, 287, 1977) proposed an initiator obtained in the presence of 1,3-diisopropenylbenzene, secondary butyllithium and 0.1 lithium equivalent of triethylamine. This initiator is obtained in the form of xcex1,xcfx89-dilithiated oligo-isoprene, in order to solubilize it in a non-polar medium. Said initiator was used for the preparation of polymers obtained anionically, and the effectively bifunctional behavior of this initiator was concluded on the basis of the number-average molecular weights and the monomodal distributions of molecular weights which were obtained for these polymers.
However, G. Gordon Cameron and G. M. Buchan (Polymer 20, 1129, 1979) report that molecular weights and distributions of molecular weights indicate only average functionality for the two carbon-lithium sites of the initiator. These elements of macrostructure do not constitute a sufficient condition for defining the equi-reactivity of these sites during polymerization, that is to say, the effective bifunctionality of the initiator.
In fact, the aforementioned monomodal nature for the distributions of molecular weights can be explained using the hypothesis that the polymerization kinetics of each of the initiating types present (i.e. residual monolithiated compound, dilithiated compound, polylithiated compounds resulting from the oligomerization of 1,3-diisopropenylbenzene) are compensated.
In the second example of implementation of the reaction between the 1,3-diisopropenylbenzene and the secondary butyllithium, U.S. Pat. No. 5,464,914 discloses the addition of 10 lithium equivalents of triethylamine to obtain a dilithiated initiator. In order to make the initiator with a bifunctional nature during polymerization, it should be noted that a polyamine must later be introduced into the polymerization medium.
In fact, the polarity provided by the triethylamine is not sufficient to dissociate the C-Li sites borne by the molecule of initiator, i.e. to reveal the bifunctional nature of the initiator.
Ph. TEYSSIE et al. discloses the preparation of a dilithiated initiator by reaction of 1,3-diisopropenylbenzene with tert. butyllithium, in stoichiometric quantities and in the presence of 1 lithium equivalent of triethylamine and at xe2x88x9220xc2x0 C. (Ph. Teyssie et al., Macromolecules 29, 2738, 1996), such that the 1 ,3-diisopropenylbenzene is added drop by drop to a tert. butyllithium/triethylamine solution.
The use of tert. butyllithium, instead of secondary butyllithium, overcomes a disadvantage inherent in the use of secondary butyllithium, i.e. the production in a large quantity of oligomers. In fact, since tert. butyllithium is more reactive than secondary butyllithium, the reaction of the 1,3-diisopropenylbenzene with the tert. butyllithium is thus preferentially oriented towards addition of the lithiated compound to each of the two unsaturated sites borne by the diisopropenylbenzene.
The size exclusion chromatography technique (xe2x80x9cSECxe2x80x9d technique) made it possible to establish the absence of oligomers of 1,3-diisopropenylbenzene, and the 7Li NMR technique demonstrated the substantially total consumption of the tert. butyllithium.
It will also be noted that the 1,3-diisopropenylbenzene/tert. butyllithium molecule thus obtained only behaves like a bifunctional initiator upon polymerization, i.e. equi-reactivity of the carbon-lithium sites, if a sufficient quantity of a polar compound, for example, tetrahydrofuran, is added.
International patent specification WO-A-00/22004 discloses a process for the preparation of a dilithiated initiator consisting of reacting diisopropenylbenzene with diethyl ether, then adding the mixture thus obtained to a secondary or tertiary alkyllithium. The reaction of said mixture with a secondary alkyllithium is effected at a temperature of between 40xc2x0 C. and 50xc2x0 C. or, with a tertiary alkyllithium, at a temperature of between 25xc2x0 C. and 50xc2x0 C.
The present invention relates to a process for the preparation of a dilithiated initiator usable in anionic polymerization, a process for the preparation by anionic polymerization of a living diene elastomer which is provided with a carbon-lithium group at each of its chain ends, and a process for the preparation of a copolymer having three blocks starting from said living diene elastomer.